The present invention relates to the control of an electric generator set including an engine and an alternator. In particular, the present invention relates to the control of an electric generator set to prevent the generation of excessive heat by the alternator.
Electric generator sets (or xe2x80x9cgensetsxe2x80x9d) are widely used to provide electric power. A genset typically includes an engine coupled to an alternator, which converts the rotational energy from the engine into electrical energy. The terminal voltage of a genset is proportional to both the magnetic flux density within the alternator, and the speed of the engine. The magnetic flux density is typically determined by controlling an armature voltage or field current on the alternator, while the speed of the engine is typically determined by an engine governor.
During the operation of the genset, excessively high levels of current can be produced within the alternator depending upon the load, the engine speed, and other factors. Such excessively high levels of current can produce excessive heat within the alternator, which can damage the alternator and produce other undesirable effects. Consequently, it is known to include a mechanism with a genset that stops or otherwise limits the operation of the genset if excessively high levels of current occur.
Typically, an alternator does not suffer damage or other undesirable effects from excessive heat unless it is exposed to the heat for an extended period of time. Consequently, excessively high currents within the wire windings of an alternator do not immediately cause damage to the alternator. Rather, exposure to excessively high currents over an extended period of time is the cause of damage to an alternator. It is known to determine whether excessive exposure is occurring in an alternator by (a) measuring the RMS current I within the alternator during a given time period, (b) taking the square of these current measurements to obtain I2 during the time period, (c) calculating an integral or summation of I2 over the time period to obtain a value I2t, (d) comparing the result of the integral or summation with a damage curve specifying the maximum allowable value for I2t during the time period, and then (e) repeating this process during successive time periods.
While it is exposure to excessively high currents over an extended period of time that precipitates damage in an alternator, the levels of the currents to which the alternator can be exposed without sustaining damage vary significantly depending upon the amount of time the alternator is exposed to those currents. In particular, an alternator can be exposed to currents that are very high in magnitude when those currents are short-term transient currents, but can only be exposed to currents that are much lower in magnitude when the alternator is exposed to those currents over a long period of time.
Because the tolerance of an alternator with respect to excessive currents varies depending upon the time the alternator is exposed to those currents, the above-discussed process is not always an accurate indicator of whether the alternator is experiencing exposure to excessive currents. On the one hand, the above process may fail to indicate that the alternator is experiencing excessive exposure to a short burst of extremely high current because the average current level during the measured period of time is such that the overall value of I2t is less than the maximum tolerance specified by the damage curve for that period of time.
On the other hand, the above process may incorrectly indicate that the alternator is experiencing excessive exposure when there is a short burst of extremely high current such that the overall value of I2t exceeds the damage curve even though, over the long term, the current levels are actually tolerable. This may be the case where, for example, the current immediately decreases to a very low level following the short burst of high current. This is particularly problematic if the damage curve is conservative, namely, the allowed values of I2t are set low. The damage curve may be set conservatively in order to avoid the problems discussed above, i.e., to minimize the inappropriate tolerance of short bursts of high current.
It would therefore be advantageous if a method and apparatus were developed for determining whether an alternator was being exposed to excessive currents and resultant heat, where the method and apparatus accounted for whether the currents were occurring for long periods of time or only for short periods of time. In particular, it would be advantageous if a method and apparatus were developed for determining excessive exposure of an alternator to currents during a given period of time in spite of the existence of significant short term variations of the currents within that period of time. It would additionally be advantageous if the method and apparatus were configured to cause a cessation or reduction in the operation of the genset upon a determination of the existence of excessive currents.
The present invention relates to a method of preventing damage to an alternator of a genset resulting from high currents within the alternator. The method includes calculating, at a processor, a first quantity related to a current flowing through the alternator during a first time period, and comparing, at the processor, the first quantity with a first threshold. The method further includes calculating, at the processor, a second quantity related to the current flowing through the alternator during at least one of the first time period and a second time period, and comparing, at the processor, the second quantity with a second threshold. The method additionally includes indicating a current overload condition if at least one of the first quantity exceeds the first threshold or the second quantity exceeds the second threshold.
The present inventors have discovered that it is possible to accurately determine whether an alternator is being exposed to excessive currents and resultant heat over a given period of time in spite of the existence of significant short term variations of the current by determining values of I2t (or related to I2t) over both short periods of time and over long or unending periods of time, and then comparing the determined values with respect to both a short term limit and a long term limit, respectively. With respect to determining the value of I2t over the long term, it is necessary to increase the overall calculated value of I2t to account for periods of time in which the current I is greater than a rated current, and to decrease the overall calculated value of I2t to account for periods of time in which the current I is less than the rated current.
The present invention additionally relates to a method of preventing damage to an alternator of a genset resulting from high currents within the alternator. The method includes (a) calculating a first quantity equaling a first sum of n squares of n type-A normalized currents determined based upon n most recent current measurements obtained during n most recent successive short time periods; (b) determining whether the first quantity exceeds a first threshold; and (c) providing a first command to limit an operation of the genset when it is determined that the first quantity exceeds the first threshold. The method further includes (d) calculating a second quantity equaling a second sum of x values functionally dependent upon x squares of x type-B normalized currents determined based upon x current measurements obtained during x successive short time periods, wherein x is the total number of short time periods that have occurred since at least one of a first time at which a performance of the method began and a second time occurring after the first time. The method additionally includes (e) determining whether the second quantity exceeds a second threshold; and (f) providing a second command to limit an operation of the genset when it is determined that the second quantity exceeds the second threshold.
The present invention further relates to an apparatus for preventing damage to an alternator of a genset resulting from high currents within the alternator. The apparatus includes a means for measuring a current flowing within the alternator during successive time periods, and a processor coupled to the means for measuring. The processor is operable to calculate a first parameter that is related to a first set of measured current values that were obtained by the means for measuring during a first time period, and to calculate a second parameter that is related to a second set of measured current values that were obtained by the means for measuring during a second time period. The processor is further operable to compare the first parameter with a first threshold and the second parameter with a second threshold, and to provide a command to cause at least a portion of the genset to be limited in its operation when either the first parameter exceeds the first threshold or the second parameter exceeds the second threshold. The apparatus further includes a means responsive to the command to alter the operation of the genset.
The present invention additionally relates to an apparatus for preventing damage to an alternator of a genset resulting from high currents within the alternator. The apparatus includes a means for determining whether a current flowing through the alternator during a recent time period has been excessive; and a means for determining whether a current flowing through the alternator during an extended time period has been excessive. The apparatus further includes a means for halting the operation of the genset if at least one of the two determining means determines that the current has been excessive.